Countersink Angle & Fastener Head Chart

What it is

A flat (countersunk) head screw sits flush only when the countersink angle matches the angle under the head. Countersink angles are given as the included angle, measured across the whole cone. The wrong angle leaves the head bearing on only its edge or only its top, so it sits high, cracks the part or loosens.

How to identify

Check the fastener standard on the print or package. Inch machine screws and inch flat head cap screws use 82 degrees. Metric countersunk screws use 90 degrees. Aircraft and many aerospace fasteners use 100 degrees. A countersink gauge or a protractor on the head confirms the angle.

Chart

Key points

• Inch and metric flat heads look alike. An 82 degree countersink under a 90 degree head bears only on the outer edge.
• Countersink depth is set by the head diameter. Cut until the head sits flush or slightly below, then check with the actual fastener.
• Single-flute and zero-flute countersinks chatter less than multi-flute tools in many materials.
• Undercut 82 degree heads (shorter heads for short screws) use the same angle but need a shallower countersink.

Common mistakes

• Using a 90 degree countersink for inch hardware because it is "close enough."
• Cutting the countersink before drilling the clearance hole, which lets the tool wander.
• Countersinking thin sheet so deep that the cone goes through the sheet. Thin material may need dimpling instead.

Governing standards: ASME B18.6.3 (machine screws, tapping screws and metallic drive screws, inch); ISO 7046-1 (countersunk flat head screws, metric); ASME B18.3 and ISO 10642 for socket head countersunk screws.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.